In recent years, in order to protect the global environment, there have been demands for decreases in the weight of automotive bodies as one way of decreasing the amount of CO2 discharged from automobiles. It is not permissible for decrease in weight to decrease the strength demanded of automotive bodies. Therefore, increases in the strength of steel sheets for automobiles are being promoted.
There are increased demands by society for safety of automobiles in collisions. For this reason, the properties demanded of steel sheets for automobiles are not simply a high strength; there is also a desire for improved impact resistance if a collision should occur during driving. Namely, there is a desire for high resistance to deformation when deformation takes place at a high strain rate. The development of steel sheets which can satisfy these demands is being studied.
In general, it is known that steel sheets made of mild steel have a large difference between the static stress and the dynamic stress (in this invention, this is referred to as the static-dynamic difference) and that the difference decreases as the strength of steel sheets increases. An example of a multi-phase steel sheet which has a large static-dynamic difference while having a high strength is a low-alloy TRIP steel sheet.
As a specific example of such a steel sheet, Patent Document 1 discloses a strain induced transformation-type high-strength steel sheet (TRIP steel sheet) having improved dynamic deformation properties which is obtained by pre-straining a steel sheet having a composition comprising, in mass percent, 0.04-0.15% C, one or both of Si and Al in a total of 0.3-3.0%, and a remainder of Fe and unavoidable impurities and having a multi-phase structure comprising a main phase of ferrite and a second phase which includes at least 3 volume percent of austenite. The pre-straining is carried out by one or both of temper rolling and straightening through a tension leveler such that the amount of plastic deformation T produced by pre-straining satisfies the following Equation (A). The steel sheet before pre-straining has such a property that the ratio V(10)/V(0) which is the ratio of the volume fraction V(10) of the austenite phase after deformation at an equivalent strain of 10% to the initial volume fraction V(0) of the austenite phase is at least 0.3. The steel sheet is characterized in that the difference (σd-σs) between the quasi-static deformation strength as when deformed at a strain rate in the range of 5×10−4−5×10−3 (s−1) after pre-straining in accordance with Equation (A) below and the dynamic deformation strength σd when deformed at a strain rate in the range of 5×102-5×103 (s−1) after the pre-straining is at least 60 MPa.0.5[{(V(10)/V(0))/C}−3]+15≥T≥0.5[{(V(10)/V(0))/C}−3]  (A)
As an example of a multi-phase steel sheet having a second phase which is primarily martensite, Patent Document 2 discloses a high-strength steel sheet having an improved balance of strength and ductility and having a static-dynamic difference of at least 170 MPa. The steel sheet comprises fine ferritic grains in which the average grain diameter ds of nanocrystal grains having a grain diameter of at most 1.2 μm and the average grain diameter dL of microcrystal grains having a grain diameter exceeding 1.2 μm satisfy dL/ds≥3. In that document, the static-dynamic difference is defined as the difference between the static deformation stress obtained at a strain rate of 0.01 s−1 and the dynamic deformation stress obtained when carrying out a tensile test at a strain rate of 1000 s−1. However, Patent Document 2 does not contain any disclosure concerning the deformation stress in an intermediate strain rate region where the strain rate is greater than 0.01 s−1 and less than 1000 s−1.
Patent Document 3 discloses a steel sheet having a high static-dynamic ratio having a dual-phase structure of martensite having an average grain diameter of at most 3 μm and ferrite having an average grain diameter of at most 5 μm. In that document, the static-dynamic ratio is defined as the ratio of the static yield stress obtained at a strain rate of 10−3 s−1 to the dynamic yield stress obtained at a strain rate of 103 s−1. However, there is no disclosure concerning the static-dynamic difference in a range in which the strain rate is greater than 0.01 s−1 and less than 1000 s−1. In addition, the static yield stress of the steel sheet disclosed in Patent Document 3 is a low value of 31.9 kgf/mm2-34.7 kgf/mm2.